The application of a hard handover has been proposed as a handover between base stations (cells) for the evolved universal terrestrial radio access network (E-UTRAN) system of a next-generation radio access network. A discussion of the application of the hard handover has been started in the Third Generation Partnership Project (3GPP). In the hard handover, a mobile station establishes a radio link with a hard handover destination base station after the mobile station disconnects a radio link with a hard handover source base station.
Accordingly, the use of hard handover makes a system less complicated in comparison with a system using a soft handover. However, in the hard handover, the handover destination base station cannot receive user data that remains stored in the handover source base station. Hence, there has been a known method in which the handover source base station transfers the user data to the handover destination base station in packet units (refer to Non-Patent Document 1, for example).
However, normal data flows through a transmission path as a traffic. Thus, when a plurality of the mobile stations exists and when the plurality of the mobile stations performs the handover and a data communication, a congestion occurs on the transmission path by normal data and user data which is transferred between base stations at the time of the handover.
Due to the congestion on the transmission path, the user data transferred at the time of the handover could be lost. As a result, the user data is retransmitted in an upper layer. Therefore, a user throughput could largely deteriorate, and a duration of an instantaneous interruption that occurs at the time of the handover could be increased.
Non-Patent Document 1: “Fast Handovers for Mobile IPv6”, RFC4068